1. Field of the Invention
This invention relates to semiconductor processing, and more particularly to the protection of the backsides and edges of wafers during chemical vapor deposition.
2. Description of Related Art
Chemical vapor deposition ("CVD") is a gas reaction process commonly used in the semiconductor industry to form thin layers of material known as films over an integrated circuit substrate. The CVD process is based on the thermal, plasma, or thermal and plasma decomposition and reaction of selected gases. The most widely used CVD films are silicon dioxide, silicon nitride, and polysilicon, although a wide variety of CVD films suitable for insulators and dielectrics, semiconductors, conductors, superconductors, and magnetics are well known.
Particulate contamination of CVD films must be avoided. A particularly troublesome source of particulates in the chemical vapor deposition of metals and other conductors such as tungsten, tungsten silicide, and titanium nitride is the film that forms on the edge and backside of the wafer under certain conditions. For example, if the wafer edge and backside are unprotected or inadequately protected during deposition, a partial coating of the CVD material forms on the wafer edge and backside, respectively. This partial coating tends to peel and flake easily for some types of materials, introducing particulates into the chamber during deposition and subsequent handling steps.
Many approaches have been developed for addressing the problem of material deposition on the wafer backside. In one approach, the material is permitted to form on the backside, but then is removed immediately following the deposition step using an in-situ plasma etch. This approach entails additional process steps and requires additional equipment capabilities, and also affects the flatness of the wafer. In another approach, the wafer is clamped onto a substrate holder in an attempt to seal and isolate the backside region from the CVD gas. An adequate seal tends to be difficult to achieve in practice, and the mechanical motion between the clamp and the wafer itself causes particulates. Yet another approach is disclosed in U.S. Pat. No. 4,817,558, issued Apr. 4, 1989 to Itoh. A substrate support member having the form of a cylinder is provided with a flat bearing surface on which the substrate rests. Three pins protrude from the peripheral edge portion of the bearing surface. The sidewalls of the substrate are insulated from the reactive gases by a cover, which is further provided with a lifted and bent region that surrounds the substrate at the level of the substrate. The lifted and bent region is said to trap the reactive gas on the lateral face of the wafer, thereby preventing a film from being deposited on the wafer backside.
The problem of edge and backside deposition is exacerbated by recent trends in the semiconductor industry toward more full utilization of the wafer surface area The industry demands that material deposition conform to stated specifications over a major portion of the surface area of the wafer. Factors specified include uniformity and quality. Typically a portion of the frontside periphery of the wafer is excused from compliance with the specification because of wafer handling requirements. For example, for a 200 mm wafer, which is known in the industry as an "eight inch" wafer, the outside 6 mm of radius typically has been excused from compliance with the deposition standard, which results in an effective usable wafer diameter of 188 mm. However, recent supply problems in the industry due to capacity limitations have resulted in some manufacturers seeking greater utilization of the wafer surface area. For example, interest has been expressed in having an effective usable wafer diameter of at least 194 mm. Accordingly, methods and apparatus for chemical vapor deposition that permit an effective usable wafer diameter of nearly the full wafer diameter while preventing unwanted edge and backside deposition are desirable.